Problem 5 - Biochemistry

Question

How does the combined activity of glutamine synthetase and glutamate synthase assimilate \(\mathrm{NH}_{4}^{+}\) into amino acids? Why doesn't the glutamate dehydrogenase reaction contribute much to ammonia assimilation under normal physiologic conditions?

Step-by-Step Solution

Answer

Question: Explain how the combined activity of glutamine synthetase and glutamate synthase assimilates NH4+ into amino acids, and why the glutamate dehydrogenase reaction doesn't contribute much to ammonia assimilation under normal physiological conditions. Answer: The combined activity of glutamine synthetase and glutamate synthase assimilates NH4+ into amino acids through two sequential reactions. First, glutamine synthetase catalyzes the ATP-dependent reaction between glutamate and ammonia to form glutamine. Second, glutamate synthase converts glutamine back to glutamate in a reaction with 2-oxoglutarate, during which, another molecule of NH4+ is assimilated into the newly formed glutamate. In contrast, under normal physiological conditions, the glutamate dehydrogenase reaction predominantly favors glutamate degradation, releasing NH4+ instead of assimilating it, due to high concentrations of glutamate and NADH, and the enzyme's low affinity for ammonia.

Step 1: Glutamine Synthetase Activity

Glutamine synthetase catalyzes the ATP-dependent reaction between glutamate and ammonia to form glutamine. The reaction is as follows: Glutamate + \(\mathrm{NH}_{4}^{+}\) + ATP → Glutamine + ADP + Pi This reaction incorporates one molecule of \(\mathrm{NH}_{4}^{+}\) into glutamate, producing glutamine.

Step 2: Glutamate Synthase Activity

Glutamate synthase converts glutamine back to glutamate in a reaction with 2-oxoglutarate, a Krebs cycle intermediate. During this conversion, another molecule of \(\mathrm{NH}_{4}^{+}\) is assimilated into the newly formed glutamate. The reaction is as follows: Glutamine + 2-oxoglutarate + NADPH + H+ → 2 Glutamate + NADP+ The combined activity of these two enzymes assimilates a total of two molecules of \(\mathrm{NH}_{4}^{+}\) into amino acids.

Step 3: Limited Contribution of Glutamate Dehydrogenase Reaction

The glutamate dehydrogenase reaction allows for the reversible conversion of glutamate to 2-oxoglutarate and ammonia, as shown in the reaction below: Glutamate + NAD+ ↔ 2-oxoglutarate + \(\mathrm{NH}_{4}^{+}\) + NADH Under normal physiological conditions, this reaction is predominantly in the direction of glutamate degradation, releasing \(\mathrm{NH}_{4}^{+}\). This occurs because the concentrations of glutamate and NADH are typically high in the cell, and the enzyme's affinity for ammonia is relatively low. Consequently, the glutamate dehydrogenase reaction does not significantly contribute to ammonia assimilation in most instances.